Leetcode17 Phone number DFS - a second practice using previous practice

Leetcode17PhoneNumberDFS.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _17LetterCombinationOfAPhoneNUmber_DFS
{
    /*
     * Leetcode 17:
     * Given a digit string, return all possible letter combinations that 
     * the number could represent.
       A mapping of digit to letters (just like on the telephone buttons) is given below.
     * Input:Digit string "23"
       Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
     * 
     * Telephone dial pad:
     *                     0  1   2     3     4     5     6     7      8     9 
     * String[] keyboard={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
     */
    class Solution
    {
        static void Main(string[] args)
        {
            IList<string> list = letterCombination("23");
            // test result: "abc", "def", so 3x3 = 9 cases. 
        }

        public static IList<string> letterCombination(string digits)
        {
            IList<string> list = new List<string>();
            if (digits == null || digits.Length == 0)
                return list;

            string[] keyboard = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

            StringBuilder sb = new StringBuilder(); 
            buildResult(list, keyboard, digits, sb, 0);  // think about design here

            return list;
        }

        /*
         * Julia's practice on April 26, 2016
         * Need to rewrite to get more experience. 
         * Static analysis: 
         * 1. use explanation variables c, s1, 
         * 
         */
        private static void buildResult(
            IList<string>   list, 
            string[]        keyboard, 
            string          digits, // test case: "23"
            StringBuilder   sb, 
            int             index)  // test case: "23", index value 0 and 1
        {
            // need to do the backtracking, need to do the recursive solution 
            int len = digits.Length;

            // index starting from 0 
            if (index < len)   // bug001 - not len -1, should be len
            {
                char    c = digits[index];
                string s1 = keyboard[c - '0'];

                foreach (char nextChar in s1)
                {
                    string tmpS = sb.ToString(); 

                    sb.Append(nextChar);                    
                    buildResult(list, keyboard, digits, sb, index + 1);

                    sb = new StringBuilder(tmpS);                     
                }
            }
            else
            {
                // add to the list
                list.Add(sb.ToString()); 
            }            
        }        
    }
}